Re: Abstract classes

"Victor Bazarov" <>
Fri, 9 Nov 2007 11:03:32 -0500
TBass wrote:

On Nov 9, 9:48 am, Brian Szmyd <> wrote:

Where are:

        virtual bool InRect(double x, double y); virtual bool
        OnLButtonDown(double x, double y); virtual bool
        OnLButtonUp(double x, double y); virtual bool
        OnRButtonDown(double x, double y); virtual bool
        OnRButtonUp(double x, double y);

Implemented? You've made them pure virtual in your parent class, but
I don't see the child class implementation of them. You can't
instantiate an object of a class that has any pure virtual methods
(inherited or not).

Ah! I thought I could pick and choose which virtual functions I wanted
to implement.

You can. The problem is that your class cannot control which functions
of it are going to be called. And calling a pure virtual function (if
you don't implement it, it stays pure, right?) has undefined behaivour.

If you want to be able to have an object that does nothing upos one of
those actions, don't make those functions pure. Provide the compiler
with an implementation in the base class, and let it return 'false' (or
whatever makes sense for "default behaviour -- nothing is done"). Then
you can pick which behaviour change from the default to custom.

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